One form of valve for a pMDI is described in WO-A-04/22142, which discloses an aerosol metering valve comprising:
(a) a valve stem that generally defines a longitudinal axis and comprises:                (1) a body portion comprising a proximal end, a distal end, and at least one side surface connecting the proximal end and the distal end and comprising a metering surface, wherein the longitudinal axis and a plane tangential to at least a portion of the metering surface define an angle from about 2° to about 90°, and        (2) a stem portion comprising a discharge passageway;        
(b) a valve body comprising:                (1) a body wall that comprises a sealing portion,        (2) an internal chamber defined at least in part by the body wall and comprising a metering portion configured to substantially conform to the metering surface of the valve stem, and        (3) a diaphragm having walls that define an aperture in slidable, sealing engagement with the stem portion of the valve stem; and        
(c) a metering gasket configured to be able to form a transient, substantially fluid-tight face seal between the valve stem, in particular the body portion of the valve stem, and the sealing portion of the body wall. As the valve stem moves from its rest position towards its firing position a surface of the body portion of the valve stem contacts the metering gasket to form a face seal thereby closing a metering chamber and thereafter further movement of the valve stem to its firing position causes the metering gasket to deflect while maintaining the seal with the aforementioned surface of the valve stem. At the rest position of the valve stem, the metering gasket is not in contact with the valve stem.
GB 1201919 discloses an aerosol metering valve having two annular seals made from elastomeric sheet material and which are constrained to adopt conical configurations when assembled into the valve. Constraining the seals in this way prevents them dishing inwardly upon operation of the valve. The seals are not caused to deflect while maintaining a seal with the valve stem upon movement of the valve stem towards its firing position.